Very close to perfect! A lot of great features! Excellent for both security and privacy. Just a couple of points keeping it from 5 stars:

1st: We need an option so that all tabs set to open in a container by default won't prompt the user to ask if they want to open a link in its assigned container. It should just do it.

2nd: "The Manage Site List..." menu needs an option to manually enter a web address. Currently the only option is to use the address bar button while navigating, but this can break some re-direct heavy sites. Slack logins, for instance, may go through several different corporate domains in an instant and will break in horribly painful ways if one of those URLs isn't allowed to open in the container holding the login cookie.

3rd: Mozilla VPN is a good idea, but its implementation would be less vulnerable to traffic profiling attacks if it were on by default on all containers and each individual container had a separate opt-out option instead of the current enable option. I'd also like if the service weren't tied to specific Firefox accounts, or had a multi-account payment plan, so that I could use it on my work and home accounts at the same time.

4th: This is just a wish-list thing, but I'd absolutely adore it if you allowed us to use third party VPN logins instead of Mozilla VPN. This would be really great for tunneling into VPNs tied to router networks in the home or at work as if the user were physically on location. A lot of companies do this for security and including this could really help adoption.

Address at least the first three of these and you'll get five stars!